The present invention relates to a method of preventing the pollution of sludge formed by an electrolytic etching of a chromium containing metal. Electrolytic etching work have been applied for processing hardly shapeable steels such as chromium containing metals e.g. stainless steels, heat resisting alloy steels and the like.
In the electrolytic etching work, the dissolved chromium formed by an anodic electrolysis under high current density is accumulated in the electrolytic solution in the state of Cr (VI) such as chromate ions by an electrolytic oxidation.
The ferric oxide formed by the electrolytic etching work is separated from the electrolytic solution by means of a centrifugal separation or a filtration in vacuum.
Since, the electrolytic solution contains the chromium (VI) compound, the sludge also contains the chromium (VI) compound, which causes the pollution of environment.
The maximum regulated value of Cr(VI) is 1.5 mg of Cr.sup.6.sup.+ per 1 liter of a sampling solution according to the leaching analysis. The sludge containing Cr(VI) of higher than said maximum regulated value should not be reclaimed under the regulation.
The sludge discharged from the conventional electrolytic etching usually contains about ten times of Cr(VI) to the maximum regulated value. Accordingly, the sludge causes the pollution of environments.
It has been proposed to remove chromate ions from an electrolytic solution in an electrolyic etching work by adding more than stoichiometric amount of ferrous sulfate to the electrolytic solution in acidic condition. (Japanese Unexamined Patent Publication No. 35061/1975).
It has been known to reduce chromate ions with ferrous ions as follows. EQU Cr.sub.2 O.sub.7.sup.2.sup.- + 14H.sup.+ + 6Fe.sup.2.sup.+ .fwdarw. 2Cr.sup.3.sup.+ + 6Fe.sup.3.sup.+ + 7H.sub.2 O.
ferrous sulfate is acidic compound.
When an electrolytic solution having pH of 11 is admixed with stoichiometric amount of ferrous sulfate, pH of the electrolytic solution decreases to about 3.2, and the reduction of chromate ions to Cr(III) is caused in acidic condition.
The sludge formed by the known method also causes the pollution, because of high leaching of Cr(VI) compound from the sludge.